This invention relates to bicycle wheel rims of the type presenting a peripheral inner wall, a peripheral outer wall, two lateral walls joining said peripheral walls and two circumferential wings for anchoring a tyre which radially extend outwards from the two sides of the outer peripheral wall.
This invention also relates to a method for producing a rim of the type described above.
The Applicant has recently conducted various studies and tests to make bicycle wheel rims using structural fibre based material, typically carbon fibre based material. The advantage offered by this type of material is that of being light in weight with respect to the metallic materials used in the past given equal structural characteristics. Making a rim out of a single part of carbon fibre based material was difficult, at least utilising the technologies available at that time, due to the typical conformation of the circumferential anchoring wings of the tyre. Typically, these wings present peripheral outer edges folded one towards the other thus creating an undercut, which causes the moulding problems.
The object of this invention is to overcome this technical problem.
In order to attain this object, this invention relates to a rim for a bicycle wheel, comprising an inner peripheral wall, an outer peripheral wall, two lateral walls joining said peripheral walls, and two circumferential wings for anchoring a tyre which radially extend outwards from the two sides of the outer peripheral wall, wherein said rim is made of a single part of structural fibre based material incorporating the two circumferential wings.
This invention relates also to a method for producing a bicycle wheel rim of the type presenting an inner peripheral wall, an outer peripheral wall, two lateral walls joining said peripheral walls , and two circumferential wings, for anchoring a tyre, which extend outwards from the two sides of the outer peripheral wall,
wherein it comprises the following steps:
applying on the inner part of a mould a predetermined number of layers of structural fibre fabric incorporated in a plastic material matrix which are to form the inner wall, the outer wall, the two lateral walls and the wings;
arranging an inflatable bag on the layers;
folding a first predetermined number of the layers on the inflatable bag;
applying at least one core over the folded first predetermined number of layers;
folding a second predetermined number of the layers over the core;
applying the outer part of the mould so as to enclose the layers;
inflating the inflatable bag so as to press the layers against the mould;
increasing the temperature of the mould to a value sufficient to cause reticulation of the plastic material matrix;
removing the bicycle wheel rim from the mould and removing the core, so as to obtain a bicycle wheel rim formed of a single piece of structural fibre material.
The cores are made of a material with a thermal dilation coefficient exceeding 5xc3x9710xe2x88x925 mm/xc2x0 C., the moulding process comprising an increase in temperature to a value sufficient to cause the material of said cores to dilate so as to press the layers of fabric forming the tyre anchoring wings against the wall of the mould.
Preferably, the material forming the cores has a thermal dilation coefficient exceeding 9xc3x9710xe2x88x925 mm/xc2x0 C. and a maximum continuous thermal resistance temperature exceeding 100xc2x0 C.
Again preferably, the material forming the core can be either PTFE (polytetrafluoroethene), or FEP (fluorinated ethene propene), or PCTFE (polychlorotrifluoroethene), or PVDF (polyfluorodivinylidene), or PE-HD (high density polyethylene).
The use of PTFE is widely preferred, due to the anti-adherence properties of this material, which are useful for detaching the core, or cores, from the structural fibre moulded body, as well as its high continuous thermal resistance (260xc2x0 C.), for its good thermal conductivity (0.25W/mxc2x0 C.) and for its good thermal capacity (specific heat), equal to 1.045 Kj/kgxc2x0 C.
This material presents the characteristic of being subject to high thermal dilation at relatively low temperatures, in the order of temperatures at which the plastic material in which the structural fibre fabric is incorporated reticulates.
Structural fibre fabrics incorporated in a plastic material matrix are known and have been used for some time. They are made with yarn obtained from structural fibres, such as carbon fibres, for example. These fabrics are then subjected to an impregnation method to associate them with a plastic material matrix, typically a thermosetting plastic material.
According to an additional preferred embodiment of this invention, two ring-shaped cores of said thermally dilating material are used, each split into several sectors if required, which are arranged so as to be spaced from each other around the layers which are to form the peripheral external wall of the rim, each core supporting one of the two tyre anchoring wings.
In a first embodiment, the space comprised between said two rings is filled by one wall of the mould. In a second form of embodiment, this space is filled by a third core, which is also a ring formed by several sectors, made of thermally dilating material. The cores being split into several sectors allows easy removal of said cores from the part obtained at the end of the moulding process, after opening the mould, despite the undercut conformation of the two tyre anchoring wings. In a third embodiment, the core is a silicone sheath made in a single continuous piece or in sectors centered by a projection of the outer mould.
As shown above, the application of the required pressure inside the mould on the layers destined to form the outer and inner peripheral walls of the rim, as well as the two lateral walls of the rim, is obtained by arranging an inflatable bag in the cavity between these layers. The bag can remain inside the part after the end of the moulding process. This technique was previously tested by the Applicant for making rims of the type above without tyre anchoring wings. This technique cannot be used in a similar fashion for making said anchoring wings in rims of the type comprising said wings. For this reason, the Applicant has additionally developed studies to create this invention.
According to an additional preferred characteristic of this invention, the rim can initially be moulded with anchoring wings which are longer than required. In this case, after the moulding process, mechanical machining is carried out to reduce the wings to the required dimensions. This method also has the advantage of preventing superficial defects, which may be present on the circumferential edges of the two wings at the end of the moulding process. This method is an invention in its own right, regardless of the type of moulding process used.
Further features of the method of the invention are indicated in the claims.
The invention is also directed to the apparatus for carrying out the above described method as well as to the bicycle wheel rim per se. The features of the apparatus and the rim of the invention are also set forth in the appended claims.